Due to the lack of reactor construction in the Western world, very few third generation reactors have been built in developed nations. In general, Generation IV designs are still in development, and might come online in the 2030s.[1]

Though the distinction is arbitrary, the improvements in reactor technology in third generation reactors are intended to result in a longer operational life (60 years of operation, extendable to 120+ years of operation prior to complete overhaul and reactor pressure vessel replacement) compared with currently used generation II reactors (designed for 40 years of operation, extendable to 80+ years of operation prior to complete overhaul and pressure vessel replacement).

The core damage frequencies for these reactors are designed to be lower than for Generation II reactors – 60 core damage events for the EPR and 3 core damage events for the ESBWR[2] per 100 million reactor-years are significantly lower than the 1,000 core damage events per 100 million reactor-years for BWR/4 generation II reactors.[2]

The third generation EPR reactor was also designed to use uranium more efficiently than older Generation II reactors, using approximately 17% less uranium per unit of electricity generated than these older reactor technologies.[3]

EPR core catching room designed to catch the corium in case of a meltdown.

Proponents of nuclear power and some who have historically been critical have acknowledged that third generation reactors as a whole are safer than older reactors. However, while there are some strong proponents of the American third generation designs that claim they are much safer than existing reactors in the US, other engineers, although not outright saying that they are not safer, are more conservative and have some specific concerns.

The Union of Concerned Scientists in 2008 referred to the EPR as the only new reactor design under consideration in the United States that "...appears to have the potential to be significantly safer and more secure against attack than today's reactors."[7]:7

There have also been issues in fabricating the precision parts necessary to maintain safe operation of these reactors, with cost overruns, broken parts, and extremely fine steel tolerances causing issues with new reactors under construction in France.[8]

The first generation III reactors were built in Japan, in the form of Advanced Boiling Water Reactors, while several others are in construction in Europe, including the EPR at Flamanville. The next third generation reactor predicted to come on line is a Westinghouse AP1000 reactor, the Sanmen Nuclear Power Station in China, which was scheduled to become operational in 2015.[9] Its completion has since been delayed until 2017.[10]

In the USA, reactor designs are certified by the Nuclear Regulatory Commission (NRC). As of October 2014[update] the commission has approved five designs, and is considering another five designs as well.[11]